The present invention relates to speech coding and more particularly to the coding of speech signals in discrete time subframes containing digitised speech samples. The present invention is applicable in particular, though not necessarily, to variable bit-rate speech coding.
In Europe, the accepted standard for digital cellular telephony is known under the acronym GSM (Global System for Mobile communications). A recent revision of the GSM standard has resulted in the specification of a new speech coding algorithm (or codec) known as Enhanced Full Rate (EFR). As with conventional speech codecs, EFR is designed to reduce the bit-rate required for an individual voice or data communication. By minimising this rate, the number of separate calls which can be multiplexed onto a given signal bandwidth is increased.
A very general illustration of the structure of a speech encoder similar to that used in EFR is shown in FIG. 1. A sampled speech signal is divided into 20 ms frames x, each containing 160 samples. Each sample is represented digitally by 16 bits. The frames are encoded in turn by first applying them to a linear predictive coder (LPC) 1 which generates for each frame a set of LPC coefficients a. These coefficients are representative of the short term redundancy in the frame.
The output from the LPC 1 comprises the LPC coefficients a and a residual signal r1 produced by removing the short term redundancy from the input speech frame using a LPC analysis filter. The residual signal is then provided to a long term predictor (LTP) 2 which generates a set of LTP parameters b which are representative of the long term redundancy in the residual signal r1, and also a residual signal s from which the long term redundancy is removed. In practice, long term prediction is a two stage process, involving (1) a first open loop estimate long term prediction is a two stage process, involving (1) a first open loop estimate of a set of LTP parameters for the entire frame and (2) a second closed loop refinement of the estimated parameters to generate a set of LTP parameters for each 40 sample subframe of the frame. The residual signal s provided by LTP 2 is in turn filtered through filters 1/A(z) and W(z) (shown commonly as block 2a in FIG. 1) to provide a weighted residual signal {tilde over (s)}. The first of these filters is an LPC synthesis filter whilst the second is a perceptual weighting filter emphasising the xe2x80x9cformantxe2x80x9d structure of the spectrum. Parameters for both filters are provided by the LPC analysis stage (block 1).
An algebraic excitation codebook 3 is used to generate excitation (or innovation) vectors c. For each 40 sample subframe (four subframes per frame), a number of different xe2x80x9ccandidatexe2x80x9d excitation vectors are applied in turn, via a scaling unit 4, to a LTP synthesis filter 5. This filter 5 receives the LTP parameters for the current subframe and introduces into the excitation vector the long term redundancy predicted by the LTP parameters. The resulting signal is then provided to a LPC synthesis filter 6 which receives the LPC coefficients for successive frames. For a given subframe, a set of LPC coefficients are generated using frame to frame interpolation and the generated coefficients are in turn applied to generate a synthesized signal ss.
The encoder of FIG. 1 differs from earlier Code Excited Linear Prediction (CELP) encoders which utilise a codebook containing a predefined set of excitation vectors. The former type of encoder instead relies upon the algebraic generation and specification of excitation vectors (see for example WO9624925) and is sometimes referred to as an Algebraic CELP or ACELP. More particularly, quantised vectors d(i) are defined which contain 10 non-zero pulses. All pulses can have the amplitudes +1 or xe2x88x921. The 40 sample positions (i=0 to 39) in a subframe are divided into 5 xe2x80x9ctracksxe2x80x9d, where each track contains two pulses (i.e. at two of the eight possible positions), as shown in the following table.
Each pair of pulse positions in a given track is encoded with 6 bits (i.e. 3 bits for each pulse giving a total of 30 bits), whilst the sign of the first pulse in the track is encoded with 1 bit (a total of 5 bits). The sign of the second pulse is not specifically encoded but rather is derived from its position relative to the first pulse. If the sample position of the second pulse is prior to that of the first pulse, then the second pulse is defined as having the opposite sign to the first pule, otherwise both pulses are defined as having the same sign. All of the 3-bit pulse positions are Gray coded in order to improve robustness against channel errors, allowing the quantised vectors to be encoded with a 35-bit algebraic code u.
In order to generate the excitation vector c(i), the quantised vector d(i) defined by the algebraic code u is filtered through a pre-filter FE(z) which enhances special spectral components in order to improve synthesized speech quality. The pre-filter (sometimes known as a xe2x80x9ccolouringxe2x80x9d filter) is defined in terms of certain of the LTP parameters generated for the subframe.
As with the conventional CELP encoder, a difference unit 7 determines the error between the synthesized signal and the input signal on a sample by sample basis (and subframe by subframe). A weighting filter 8 is then used to weight the error signal to take account of human audio perception. For a given subframe, a search unit 9 selects a suitable excitation vector {c(i) where i=0 to 39}, from the set of candidate vectors generated by the algebraic codebook 3, by identifying the vector which minimises the weighted mean square error. This process is commonly known as xe2x80x9cvector quantisationxe2x80x9d.
As already noted, the excitation vectors are multiplied at the scaling unit 4 by a gain gc. A gain value is selected which results in the scaled excitation vector having an energy equal to the energy of the weighted residual signal {tilde over (s)} provided by the LTP 2. The gain is given by:                               g          c                =                                                            s                ~                            T                        ⁢                          Hc              ⁡                              (                i                )                                                                                        c                ⁡                                  (                  i                  )                                            T                        ⁢                          H              T                        ⁢                          Hc              ⁡                              (                i                )                                                                        (        1        )            
where H is the linear prediction model (LTP and LPC) impulse response matrix.
It is necessary to incorporate gain information into the encoded speech subframe, together with the algebraic code defining the excitation vector, to enable the subframe to be accurately reconstructed. However, rather than incorporating the gain gc directly, a predicted gain ĝc is generated in a processing unit 10 from previous speech subframes, and a correction factor determined in a unit 11, i.e.:
xcex3gc=gc/ĝcxe2x80x83xe2x80x83(2)
The correction factor is then quantised using vector quantisation with a gain correction factor codebook comprising 5-bit code vectors. It is the index vector vxcex3identifying the quantised gain correction factor {circumflex over (xcex3)}gc which is incorporated into the encoded frame. Assuming that the gain gc varies little from frame to frame, xcex3gc≅1 and can be accurately quantised with a relatively short codebook.
In practice, the predicted gain ĝc is derived using a moving average (MA) prediction with fixed coefficients. A 4th order MA prediction is performed on the excitation energy as follows. Let E(n) be the mean-removed excitation energy (in dB) at subframe n, given by:                               E          ⁡                      (            n            )                          =                              10            ⁢                          xe2x80x83                        ⁢            log            ⁢                          xe2x80x83                        ⁢                          (                                                1                  N                                ⁢                                  g                  c                  2                                ⁢                                                      ∑                                          i                      =                      0                                                              N                      -                      1                                                        ⁢                                                            c                      2                                        ⁡                                          (                      i                      )                                                                                  )                                -                      E            _                                              (        3        )            
where N=40 is the subframe size, c(i) is the excitation vector (including pre-filtering), and {overscore (E)}=36 dB is a predetermined mean of the typical excitation energy.
The energy for the subframe n can be predicted by:                                           E            ^                    ⁡                      (            n            )                          =                              ∑                          i              =              1                        4                    ⁢                      xe2x80x83                    ⁢                                    b              i                        ⁢                                          R                ^                            ⁡                              (                                  n                  -                  i                                )                                                                        (        4        )            
where [b1b2b3b4]=[0.68 0.58 0.34 0.19] are the MA prediction coefficients, and {circumflex over (R)}(j) is the error in the predicted energy Ê(j) at subframe j. The error for the current subframe is calculated, for use in processing the subsequent subframe, according to the equation:
{circumflex over (R)}(n)=E(n)xe2x88x92Ê(n)xe2x80x83xe2x80x83(5)
The predicted energy can be used to compute the predicted gain ĝc by substituting Ê(n) for E(n) in equation (3) to give:
ĝc=100.05(Ê(n)+{overscore (E)}xe2x88x92Ec)xe2x80x83xe2x80x83(6)
where                               E          c                =                  10          ⁢                      xe2x80x83                    ⁢          log          ⁢                      xe2x80x83                    ⁢                      (                                          1                N                            ⁢                                                ∑                                      i                    =                    0                                                        N                    -                    1                                                  ⁢                                  xe2x80x83                                ⁢                                                      c                    2                                    ⁡                                      (                    i                    )                                                                        )                                              (        7        )            
is the energy of the excitation vector c(i).
The gain correction factor codebook search is performed to identify the quantised gain correction factor {circumflex over (xcex3g)}c which minimises the error:
eQ=(gcxe2x88x92{circumflex over (xcex3)}gcĝc)2.xe2x80x83xe2x80x83(8)
The encoded frame comprises the LPC coefficients, the LTP parameters, the algebraic code defining the excitation vector, and the quantised gain correction factor codebook index. Prior to transmission, further encoding is carried out on certain of the coding parameters in a coding and multiplexing unit 12. In particular, the LPC coefficients are converted into a corresponding number of line spectral pair (LSP) coefficients as described in xe2x80x98Efficient Vector Quantisation of LPC Parameters at 24 Bits/Framexe2x80x99, Kuldip K. P. and Bishnu S. A., IEEE Trans. Speech and Audio Processing, Vol 1, No 1, January 1993. The entire coded frame is also encoded to provide for error detection and correction. The codec specified for GSM Phase 2 encodes each speech frame with exactly the same number of bits, i.e. 244, rising to 456 after the introduction of convolution coding and the addition of cyclic redundancy check bits.
FIG. 2 shows the general structure of an ACELP decoder, suitable for decoding signals encoded with the encoder of FIG. 1. A demultiplexer 13 separates a received encoded signal into its various components. An algebraic codebook 14, identical to the codebook 3 at the encoder, determines the code vector specified by the 35-bit algebraic code in the received coded signal and pre-filters (using the LTP parameters) this to generate the excitation vector. A gain correction factor is determined from a gain correction factor codebook, using the received quantised gain correction factor, and this is used in block 15 to correct the predicted gain derived from previously decoded subframes and determined in block 16. The excitation vector is multiplied at block 17 by the corrected gain before applying the product to an LTP synthesis filter 18 and a LPC synthesis filter 19. The LTP and LPC filters receive respectively the LTP parameters and LPC coefficients conveyed by the coded signal and reintroduce long term and short term redundancy into the excitation vector.
Speech is by its very nature variable, including periods of high and low activity and often relative silence. The use of fixed bit-rate coding may therefore be wasteful of bandwidth resources. A number of speech codecs have been proposed which vary the coding bit rate frame by frame or subframe by subframe. For example, U.S. Pat. No. 5,657,420 proposes a speech codec for use in the US CDMA system and in which the coding bit-rate for a frame is selected from a number of possible rates depending upon the level of speech activity in the frame.
With regard to the ACELP codec, it has been proposed to classify speech signal subframes into two or more classes and to encode the different classes using different algebraic codebooks. More particularly, subframes for which the weighted residual signal {tilde over (s)} varies only slowly with time may be coded using code vectors d(i) having relatively few pulses (e.g. 2) whilst subframes for which the weighted residual signal varies relatively quickly may be coded using code vectors d(i) having a relatively large number of pulses (e.g. 10).
With reference to equation (7) above, a change in the number of excitation pulses in the code vector d(i) from for example 10 to 2 will result in a corresponding reduction in the energy of the excitation vector c(i). As the energy prediction of equation (4) is based on previous subframes, the prediction is likely to be poor following such a large reduction in the number of excitation pulses. This in turn will result in a relatively large error in the predicted gain ĝc, causing the gain correction factor to vary widely across the speech signal. In order to be able to accurately quantise this widely varying gain correction factor, the gain correction factor quantisation table must be relatively large, requiring a correspondingly long codebook index vxcex3, e.g. 5 bits. This adds extra bits to the coded subframe data.
It will be appreciated that large errors in the predicted gain may also arise in CELP encoders, where the energy of the code vectors d(i) varies widely from frame to frame, requiring a similarly large codebook for quantising the gain correction factor.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantage of the existing variable rate codecs.
According to a first aspect of the present invention there is provided method of coding a speech signal which signal comprises a sequence of subframes containing digitised speech samples, the method comprising, for each subframe:
(a) selecting a quantised vector d(i) comprising at least one pulse, wherein the number m and position of pulses in the vector d(i) may vary between subframes;
(b) determining a gain value gc for scaling the amplitude of the quantised vector d(i) or of a further vector c(i) derived from the quantised vector d(i), wherein the scaled vector synthesizes a weighted residual signal {tilde over (s)};
(c) determining a scaling factor k which is a function of the ratio of a predetermined energy level to the energy in the quantised vector d(i);
(d) determining a predicted gain value ĝc on the basis of one or more previously processed subframes, and as a function of the energy Ec of the quantised vector d(i) or said further vector c(i) when the amplitude of the vector is scaled by said scaling factor k; and
(e) determining a quantised gain correction factor {circumflex over (xcex3)}gc using said gain value gc and said predicted gain value ĝc.
By scaling the energy of the excitation vector as set out above, the present invention achieves an improvement in the accuracy of the predicted gain value ĝc when the number of pulses (or energy) present in the quantised vector d(i) varies from subframe to subframe. This in turn reduces the range of the gain correction factor {circumflex over (xcex3)}gc and enables accurate quantisation thereof with a smaller quantisation codebook than heretofore. The use of a smaller codebook reduces the bit length of the vector required to index the codebook. Alternatively, an improvement in quantisation accuracy may be achieved with the same size of codebook as has heretofore been used.
In one embodiment of the present invention, the number m of pulses in the vector d(i) depends upon the nature of the subframe speech signal. In another alternative embodiment, the number m of pulses is determined by system requirements or properties. For example, where the coded signal is to be transmitted over a transmission channel, the number of pulses may be small when channel interference is high thus allowing more protection bits to be added to the signal. When channel interference is low, and the signal requires fewer protection bits, the number of pulses in the vector may be increased.
Preferably, the method of the present invention is a variable bit-rate coding method and comprises generating said weighted residual signal {tilde over (s)} by substantially removing long term and short term redundancy from the speech signal subframe, classifying the speech signal subframe according to the energy contained in the weighted residual signal {tilde over (s)}, and using the classification to determine the number of pulses m in the quantised vector d(i).
Preferably, the method comprises generating a set of linear predictive coding (LPC) coefficients a for each frame and a set of long term prediction (LTP) parameters b for each subframe, wherein a frame comprises a plurality of speech subframes, and producing a coded speech signal on the basis of the LPC coefficients, the LTP parameters, the quantised vector d(i), and the quantised gain correction factor {circumflex over (xcex3)}gc.
Preferably, the quantised vector d(i) is defined by an algebraic code u which code is incorporated into the coded speech signal.
Preferably, the gain value gc is used to scale said further vector c(i), and that further vector is generated by filtering the quantised vector d(i).
Preferably, the predicted gain value is determined according to the equation:
ĝc=100.05(Ê(n)+{overscore (E)}xe2x88x92Ec)
where {overscore (E)} is a constant and Ê(n) is the prediction of the energy in the current subframe determined on the basis of previous subframes. The predicted energy may be determined using the equation:             E      ^        ⁢          (      n      )        =            ∑              i        =        1            4        ⁢          xe2x80x83        ⁢                  b        i            ⁢                        R          ^                ⁢                  (                      n            -            i                    )                    
where bi are the moving average prediction coefficients, p is the prediction order, and {circumflex over (R)}(j) is the error in the predicted energy Ê(j) at previous subframe j given by:
{circumflex over (R)}(n)=E(n)xe2x88x92Ê(n)
The term Ec is determined using the equation:       E    c    =      10    ⁢          xe2x80x83        ⁢    log    ⁢          xe2x80x83        ⁢          (                        1          N                ⁢                              ∑                          i              =              0                                      N              -              1                                ⁢                      xe2x80x83                    ⁢                                    (                              kc                ⁢                                  (                  i                  )                                            )                        2                              )      
where N is the number of samples in the subframe. Preferably:   k  =            M      m      
where M is the maximum permissible number of pulses in the quantised vector d(i).
Preferably, the quantisation vector d(i) comprises two or more pulses, where all of the pulses have the same amplitude.
Preferably, step (d) comprises searching a gain correction factor codebook to determine the quantised gain correction factor {circumflex over (xcex3)}gc which minimises the error:
eQ=(gcxe2x88x92{circumflex over (xcex3)}gcĝc)2
and encoding the codebook index for the identified quantised gain correction factor.
According to a second aspect of the present invention there is provided a method of decoding a sequence of coded subframes of a digitised sampled speech signal, the method comprising for each subframe:
(a) recovering from the coded signal a quantised vector d(i) comprising at least one pulse, wherein the number m and position of pulses in the vector d(i) may vary between subframes;
(b) recovering from the coded signal a quantised gain correction factor {circumflex over (xcex3)}gc;
(c) determining a scaling factor k which is a function of the ratio of a predetermined energy level to the energy in the quantised vector d(i);
(d) determining a predicted gain value ĝc on the basis of one or more previously processed subframes, and as a function of the energy Ec of the quantised vector d(i) or a further vector c(i) derived from d(i), when the amplitude of the vector is scaled by said scaling factor k; and
(e) correcting the predicted gain value ĝc using the quantised gain correction factor {circumflex over (xcex3)}gc to provide a corrected gain value gc; and
(f) scaling the quantised vector d(i) or said further vector c(i) using the gain value gc to generate an excitation vector synthesizing a residual signal {tilde over (s)} remaining in the original subframe speech signal after removal of substantially redundant information therefrom.
Preferably, each coded subframe of the received signal comprises an algebraic code u defining the quantised vector d(i) and an index addressing a quantised gain correction factor codebook from where the quantised gain correction factor {circumflex over (xcex3)}gc is obtained.
According to a third aspect of the present invention there is provided apparatus for coding a speech signal which signal comprises a sequence of subframes containing digitised speech samples, the apparatus having means for coding each of said subframes in turn, which means comprises:
vector selecting means for selecting a quantised vector d(i) comprising at least one pulse, wherein the number m and position of pulses in the vector d(i) may vary between subframes;
first signal processing means for determining a gain value gc for scaling the amplitude of the quantised vector d(i) or a further vector c(i) derived from the quantised vector d(i), wherein the scaled vector synthesizes a weighted residual signal {tilde over (s)};
second signal processing means for determining a scaling factor k which is a function of the ratio of a predetermined energy level to the energy in the quantised vector d(i);
third signal processing means for determining a predicted gain value ĝc on the basis of one or more previously processed subframes, and as a function of the energy Ec of the quantised vector d(i) or said further vector c(i), when the amplitude of the vector is scaled by said scaling factor k; and
fourth signal processing means for determining a quantised gain correction factor {circumflex over (xcex3)}gc using said gain value gc and said predicted gain value ĝc.
According to a fourth aspect of the present invention there is provided apparatus for decoding a sequence of coded subframes of a digitised sampled speech signal, the apparatus having means for decoding each of said subframes in turn, the means comprising:
first signal processing means for recovering from the coded signal a quantised vector d(i) comprising at least one pulse, wherein the number m and position of pulses in the vector d(i) may vary between subframes;
second signal processing means for recovering from the coded signal a quantised gain correction factor {circumflex over (xcex3)}gc;
third signal processing means for determining a scaling factor k which is a function of the ratio of a predetermined energy level to the energy in the quantised vector d(i);
fourth signal processing means for determining a predicted gain value ĝc on the basis of one or more previously processed subframes, and as a function of the energy Ec of the quantised vector d(i) or a further vector c(i) derived from the quantised vector, when the amplitude of the vector is scaled by said scaling factor k; and
correcting means for correcting the predicted gain value ĝc using the quantised gain correction factor {circumflex over (xcex3)}gc to provide a corrected gain value gc; and
scaling means for scaling the quantised vector d(i) or said further vector c(i) using the gain value gc to generate an excitation vector synthesizing a residual signal {tilde over (s)} remaining in the original subframe speech signal after removal of substantially redundant information therefrom.